Production of cyanubic acid



Stat

Unite This invention relates to a new and improved process for the production of cyanuric acid by heating urea in a liquid.

In the production of cyanuric acid by heating molten urea, if desired in the presence of zinc chloride, ammonium chloride or other catalysts, a grey product is obtained which is highly contaminated by by-products. These impurities are less if, in accordance with a known process, the urea is introduced and finely dispersed in hot liquids, such as molten paralfin wax, parafi'in oil, molten natural wax or tetrachlorbenzene, which do not dissolve or attack urea or cyanuric acid at a temperature at which cyanuric acid forms. The urea is thereby heated very rapidlyand the formation of by-products is restricted. The extent to which the formation of lay-products is suppressed is however insuflicient. Furthermore, in continuous operation, the liquids are cracked in the course of time with the formation of low-boiling compounds which distil off, and with the formation of difiiculty volatile compounds which contaminate the cyanuric acid.

By the said process there is obtained a crude cyanuric acid of which the purity may vary between about 92% and about 47%, for example, a 92% cyanuric acid being obtained in a yield of about 79% of the theory. This degree of purity is not sufiicient for certain purposes, for example the production of trichlorocyanuric acid. Cya- 'nuric acid to be used for such purposes must first be purified, for example, by recrystallization from water or organic solvents.

One object of the invention is to provide in superior yields a very pure cyanuric acid from urea. Another object of the invention is to provide a process for the production of cyanuric acid from ureaby using substances which are highly stable under the conditions of the reac- 'achieved by introducing solid or molten urea into a hot liquid which substantially consists of diphenyl or diphenyl oxide or a corresponding compound containing up to two chlorine or bromine substituents in each of the phenyl nuclei, the temperature of the said liquid being maintained at220 to 300 C. We prefer to introduce the urea in the solid state to avoid the additional operation of melting. The weight ratio of the urea to the hot liquid may be varied within Wide limits, the preferred ratio being from 0.2 to parts of urea to 1 part of liquid. The. period of time given to the introduction of the urea into the liquid may also be varied, for example from 25 minutes to hours. By carrying out the production of cyanuric acid in a diphenyl compound in the manner described, colorless cyanuric acid is obtained in a yield of at least 95 of the theory and a purity of at least 95%.

, 3,051,708 Patented Aug, 28, 1962 2 Diphenyl, diphenyl oxide and the said halogen substitution products have the furtheradvantage that they do not undergo change at a temperature between 220 and 300 C. even over a prolonged period of time in continuous operation.

Mixtures of the said high-boiling liquids may also be used, any relative proportions in the mixture being possible. It is advantageous to use mixtures of which the melting range lies at about room temperature, since liquids are easier to handle than solids. For example, containers can more easily be filled or emptied when liquids are used. Mixtures of which the melting range lies at about room temperature can be obtained, for examplatrom diphenyl or diphenyl oxide and ahalogen diphenyl or a halogen diphenyl oxide. For example, a mixture of 80% of diphenyl oxide and 20% of diphenyl has a melting range of about 18 to 20 C.

The temperature range which is preferred for the production of cyanuric acid in accordance with the present process is about 220 to about 300 C., and it is advantageous to work at 240 to 270. Below 220 C. and above 300 C., by-products are formed which contaminate the cyanuric acid too greatly. The reaction is preferably carried out at atmospheric or reduced pressure and in continuous or discontinuous operation. Reaction at slightly elevated pressure is also possible. However, when elevated pressure is applied, there exists the danger that the cyanuric acid becomes increasinglycontaminated with melamine. Bychoosing the reaction temperature in the vicinity of the boiling point of the liquid used under the pressure employed, clogging of the distillation column, through which the ammonia formed by the reaction is withdrawn, is avoided, because the condensate reflux formed therein rinses back into the reaction vessel any substance sublimed into the distillation column. I

In order to achieve sufiicient reflux in the distillation column it is not necessary that the liquid should be heated to the boil. In the production of cyanuric acid, formation of gases, i.e. ammonia and carbon dioxide, occurs even at temperatures below the boiling point of the liquid, and the gas bubbles which rise in the column entrain vapor.

If the diphenyl compound employed boils above the preferred upper temperature limit of 300 C., the pressure may be reduced in order to lower the boiling point below 300 C. For example, diphenyl boils at about 254 to 255 C. at 760 mm. Hg, diphenyl oxide (diphenyl ether) at about 259 C. at 760 mm. Hg, 2-bromo-diphenyl at about 296 to 298 C. at 7 mm. Hg, 4-bromo-diphenyl at about 310 C. at 760 mm. Hg, 2-chloro-diphenyl at about 273 to 274 C. at 760 mm. Hg, 3-chloro-diphenyl at about 284 to 285 C. at 760 mm. Hg, 4-chloro-diphenyl at about 291 C. at 745 mm. Hg, 4,4'-dichloro-diphenyl at about 355 to 360 C. at 760 mm. Hg. The boiling ranges of the mixtures can easily be calculated from the boiling points of the components, since these compounds do not 800 parts of a mixture of the isomers of the three monochlorodiphenyls of the boiling range 270-275 C. (such 20 C.) and working as is obtained by techm'eal chlorination of diphenyl) mixed with 200 parts of diphenyl oxide in a heated container provided with a distillation column and a dephlegmator. While stirring vigorously, there is then allowed to flow in at a reaction temperature of 250 C. and a pressure of 700mm. Hg, at which the boiling range is 2 60- 265 C., 900 parts of molten urea during the course of seven hours. 250 parts of ammonia is evolved. Thecyanuric acid formed is then separated from the bulk of the liquid by filtration. The remainder of the liquid adhering .to the cyanuric acid is removed byheatin'g t 160 C. at a pressure of S;mm.- Hg.- j V r 1630 parts (97.5%) of 98% pure 'cyanuric acid is ob tained: :T eipto uct is colorless. j,

Similar results are obtained .by using, "in the form of =Jheated "liquids; dipheuyl,;chlorinati on products of. di-

phenyl oxide, a mixture of 80% of-diphenyl oxide and of diphenyl (melting range of the mixture 18- under the conditions described in Example L- i IfExam Ze' -Zi 'ZOOjpartS, diphenylokide is charged into a container .providedvwith a distillationfcolumnand a dephlegmator. Th'en atareactiontemperature 'of2581-C. and 'atmos-' ppljeric pressure, 15.40.;pa ts .of molten urea per hour, is allowed 'toflow 'in 'while stirring well. 382 parts (97% The product has the following composition Found: C, 27.8%; H, 2.5%; O,-. 37.3%'; N, 32.8%.

Calculated for C H O3N C, 27.9%; H, 2.4%; O, 37.2%; N, 32.6%.

I V Example fi 2,700 parts of diphenyl oxide are fed into a container 7 equipped with a homogenizing device and a rectifying column with dephle'gmator, and heated to 260 C. 540

- centrifuging, any remaindersof diphenyl-oxide being removed by vacuum distillation. I

There are obtained 377 parts of colorless pure cyanuric acid. 1 "V r Examp le 6 500 parts of 'diphenyl are placed in a container of th :type'described in-Example 5,and heatedto 255 C.

I 540 partsqf molten urea are added to the diphenyl'charge 4 per hour of cyanuriqacidjis recovered by continuous withdrawal of'tlie suspension from ,the-. container. and V separation ofqthe cyanuric acid ina centrifuge; J The diphenyl oxide flowing tram the centrifuge is V returned to thecontainer, 7 The'eyanuricjacid is 'freed from'jthe last traces otdiphenyloxideby steam distillation. The

Iai henyl oxi e. 'th issen a ed "a so ,fl wshack in t e containen The cyan'uriej acid. obtained, by this process is color'less, and has .a-purity 'o f 9 8%.

If the small losses of. diphenyl oxide, which occur although 'the diphenyl xid is lar y. r y led. ale nade 3 9 by d fresh.

over a period -of j-1 hour and emulsified iin-v the liquid. The'cyanuricacid -formed-is then separated from the bulk of the liquid by filtration. 'The remainder of the liquid, whichadheres 'to the cyanuric acid is'removed by vacuum distillation: I

*There are obtained 374 partspf colorlesspure-cyanuric 32-0parts1ofdiphe'nyl are placed in a container of the type described in Example 5 and heated 'to,255". C.

7 1,600 parts of granulated ureaare added'to the diphenyl charge over a period of lOhours whilethoroughly hov mogenizing the reac tion miXtHre, and finely' dispersed 200 and, 235" C. in a vessel. of l.'5 1iters capacity provided witha reflux condenser and a stirrer. .To'the hot mixturethere are added over a 2% "hour period while 's rina '90 e-f o't mi a ea fta' r he to has been added, "the solid substanceis filtered 'off from "the solution, slurried in toluene, again ffiltered, slurried in water atl,2 0, Q, filtered onc'e more and then dried at 100 T e a ei t amimi id'inih t m jofa white powder. That isayield 0t 95% of the theory.

I i The. product has the followingcompositipnf 1swamamateur: i 7:91 2 1 3.712%;

- fjExa pleq i;

2% hour periodiwhile stirring 100 'gQ'of molten urea in therein. The cyanuricjacid f ormedi'is then separated from the-bulk, of the'g'diphenyl by. centrifuging. The remainderjof thejliquid which adheres 'to'the cyan-uric acid isr emoved by vacuum distillationf i .There'afre obtained 1,120 parts of colorless pure cyanuricacid. l ff "Ihis.application is a continuation-in-part of our ap plication Serial Numher,798,335, filedyMarch 10, 1959,

now abandoned, WeclaiinzQ In a, process for the production of cyanuric acid by heating urea in an inert liquid, thedmprove'ment 'which. comprises introducingfureainto ahotiliquid meldi m qn ist ng ess ntiall of atlleastone diphenylcompound. selected from the group consisting T of, diphenyl,

diphenyl oxide andjlthesame compounds containing .up'

i h c eu bs tue s sele dfmm the. group the hot liquid is maintainedrat;ra;teniperature of from 3. The improved process as claimed 'in'claimil where :inlhehot liquid is maintained ata temperature within such a way that the individual portions are melted shortly f rth-awhite powder. uThatei's a. yield f. 95% of the r a temPeIatllfe; g l 8 1d. undenapressure such that -thejliquid is about atitsbQiling-p'oint.

4. The improved processlas claimed in claim I wherein lthe; urea is'intrqduced over aperiod-of from 25 minutes ItO..1O.-h urs..,- A a 5..'1 heimp'roved{ rocessasclaimed in claim 1 where- -in -from 0.2 to 5 parts -of urea. are introduced for every .part ofhot liquid medium... 5 r t -6. In a. process for :theflprfoductiomof cyanurie'acid by -heating urea in an inert liquid," the improvement which comprises heating a liquid medium consisting essentially of at least one diphenyl compound selected from the group consisting of diphenyl, diphenyl oxide and the same compounds containing up to two halogen substituents selected from the group consisting of bromine and chlorine in each of the phenyl nuclei to a temperature of from about 220 C. to 300 C., and While maintaining said temperature introducing urea over a period of about 25 minutes to 10 hours into the hot liquid medium, a total of from 0.2 to 5 parts of said urea being introduced for every part by weight of said liquid medium.

References Cited in the file of this patent UNITED STATES PATENTS Christmann et a1 Feb. 4, 1958 Oehlschlager Feb. 3, 1959 Perret Sept. 13, 1960 FOREIGN PATENTS Canada Sept. 28, 1954 

1. IN A PROCESS FOR THE PRODUCTION OF CYANURIC ACID BY HEATING UREA IN AN INERT LIQUID, THE IMPROVEMENT WHICH COMPRISES INTRODUCING UREA INTO A HOT LIQUID MEDIUM CONSISTING ESSENTIALLY OF AT LEAST ONE DIPHENYL COMPOUND SELECTED FROM THE GROUP CONSISTING OF DIPHENYL, DIPHENYL OXIDE AND THE SAME COMPOUNDS CONTAINING UP TO TWO HALOGEN SUBSTITUENTS SELECTED FROM THE GROUP CONSISTING OF BROMINE AND CHLORINE IN EACH OF THE PHENYL NUCLEI, SAID HOT LIQUID BEING MAINTAINED AT A TEMPERATURE OF FROM 220* C, TO 300* C, AND BEING HEATED TO SAID TEMPERATURE PRIOR TO THE INTRODUCTION OF SAID UREA. 